The present invention relates to a method of heat-treating a nitride compound semiconductor layer and a method of producing a semiconductor device.
In recent years, gallium nitride compound semiconductors such as GaN, an AlGaN compound crystal and an AlInGaN compound crystal are considered promising as component materials for semiconductor devices that can emit light in the range of from a visible light region to an ultraviolet light region. Particularly, since a light emitting diode (LED) using a gallium nitride compound semiconductor has come to be commercially used, the gallium nitride compound semiconductor devices have been of great interest. Further, a semiconductor laser (laser diode, LD) using a gallium nitride compound semiconductor has been reportedly realized as well, and applications thereof including a light source for an optical disk are expected.
When a gallium nitride compound semiconductor layer doped with a p-type impurity is formed by a vapor deposition method, the gallium nitride compound semiconductor layer formed by such a method does not become a p-type, but it constitutes a semi-insulating layer having a high resistance of at least 108 xcexa9xc2x7cm or more, i.e., an i-type compound semiconductor layer.
There is known a method of decreasing the resistance of the above i-type compound semiconductor layer having a high resistance to convert it to a p-type compound semiconductor layer, as is disclosed, for example, in JP-A-2-257679. In the method disclosed in JP-A-2-257679, the surface of an i-type gallium nitride compound semiconductor layer obtained by doping the layer with Mg as a p-type impurity is irradiated with electron beam to decrease the resistance of the surface of the gallium nitride compound semiconductor layer. In the above method, however, it is only of the surface of the gallium nitride compound semiconductor layer that the resistance can be decreased, and the above treatment takes a long time due to scanning with electron beam. Further, there is involved a problem that it is difficult to uniformly decrease the resistance of the gallium nitride compound semiconductor layer in a wafer plane.
Published Japanese Patent No. 2540791 discloses a technique of growing a gallium nitride compound semiconductor doped with a p-type impurity by a vapor deposition method, and heat-treating the semiconductor at a temperature of 400xc2x0 C. or higher or at a temperature of 600xc2x0 C. or higher for attaining a practical carrier concentration. The above heat-treatment is carried out in vacuum or an inert gas atmosphere free of hydrogen atoms of NH3 or H2 for approximately 10 to 20 minutes.
In production of a semiconductor laser (LD), however, diffusion of a p-type impurity such as Mg is presumably more liable to take place, or the sharpness of an interface in a superlattice structure is presumably more liable to be disintegrated by the diffusion of In with an increase in the heat treatment temperature, and the deterioration of an active layer such as an increase in threshold voltage Ith and a decrease in lifetime is liable to proceed easily.
Further, when the heat treatment is carried out at a high temperature, deterioration takes place in the surface of the gallium nitride compound semiconductor layer due to dissociation of nitrogen atoms. For preventing the above phenomenon, Published Japanese Patent No. 2540791 also discloses a technique for forming a cap layer on the surface of a gallium nitride compound semiconductor layer. However, the material for constituting the cap layer is selected from GaxAl1-xN (0xe2x89xa6xxe2x89xa61), AlN, Si3N4 or SiO2, and finally, it is required to remove the cap layer made of such a material from the surface of the gallium nitride compound semiconductor layer, which results in a problem that the number of manufacturing steps increases.
It is therefore an object of the present invention to provide a method of heat-treating a nitride compound semiconductor layer in which the resistance of a nitride compound semiconductor layer doped with a p-type impurity can be decreased, and the nitride compound semiconductor layer can be activated, at a far lower temperature than a temperature employed by any conventional method, and a method of producing a semiconductor device by applying the above method of heat-treating a nitride compound semiconductor layer.
According to a first aspect of the present invention for achieving the above object, there is provided a method of heat-treating a nitride compound semiconductor layer, which method comprises heating a nitride compound semiconductor layer doped with a p-type impurity at a temperature that is at least 200xc2x0 C. but less than 400xc2x0 C., preferably that is at least 225xc2x0 C. but less than 400xc2x0 C., more preferably that is at least 250xc2x0 C. but less than 400xc2x0 C., still more preferably that is at least 300xc2x0 C. but less than 400xc2x0 C., for at least 100 minutes, preferably for at least 200 minutes, more preferably for at least 500 minutes, still more preferably at least 20 hours, yet more preferably for at least 30 hours, far more preferably for at least 3xc3x97103 minutes (50 hours), further more preferably for at least 1xc3x97102 hours.
According to a first aspect of the present invention for achieving the above object, there is also provided a method of producing a semiconductor device, which method includes the step of heating a nitride compound semiconductor layer, which step comprises heating a nitride compound semiconductor layer doped with a p-type impurity at a temperature that is at least 200xc2x0 C. but less than 400xc2x0 C., preferably that is at least 225xc2x0 C. but less than 400xc2x0 C., more preferably that is at least 250xc2x0 C. but less than 400xc2x0 C., still more preferably that is at least 300xc2x0 C. but less than 400xc2x0 C., for at least 100 minutes, preferably for at least 200 minutes, more preferably for at least 500 minutes, still more preferably at least 20 hours, yet more preferably for at least 30 hours, far more preferably for at least 3xc3x97103 minutes (50 hours), further more preferably for at least 1xc3x97102 hours.
According to a second aspect of the present invention for achieving the above object, there is provided a method of heat-treating a nitride compound semiconductor layer, which method comprises heat-treating a nitride compound semiconductor layer doped with a p-type impurity in a state where the heating time period t (unit: minute) and the heating temperature T (unit: K) satisfy conditions of txe2x89xa7100, preferably txe2x89xa7200, more preferably txe2x89xa7500, still more preferably txe2x89xa72xc3x97103 and the following equation (1),
Txe2x89xa7xcex1/[ln({square root over (t)})+ln(D0)xe2x88x92ln(C)]xe2x80x83xe2x80x83(1) 
wherein xcex1 is a coefficient that is 1.04xc3x97104, ln(D0) is a coefficient that is 53 and C is a carrier concentration (unit: cmxe2x88x923) of the nitride compound semiconductor layer after the heat treatment thereof.
According to a second aspect of the present invention for achieving the above object, there is also provided a method of producing a semiconductor device, which method includes the step of heat-treating a nitride compound semiconductor layer doped with a p-type impurity in a state where the heating time period t (unit: minute) and the heating temperature T (unit: K) satisfy conditions of txe2x89xa7100, preferably txe2x89xa7200, more preferably txe2x89xa7500, still more preferably txe2x89xa72xc3x97103 and the abovementioned equation (1).
In the method of heat-treating a nitride compound semiconductor layer or the method of producing a semiconductor device according to the second aspect of the present invention (these methods will be sometimes generally referred to as xe2x80x9cmethod according to the second aspect of the present inventionxe2x80x9d hereinafter), desirably, the heating temperature T (K) is brought into a state where the condition of 473 (K)xe2x89xa6T less than 673 (K), preferably 498 (K)xe2x89xa6T less than 673 (K), more preferably 523 (K)xe2x89xa6T less than 673 (K), still more preferably 573 (K)xe2x89xa6T less than 673 (K) is satisfied. When the heating temperature T is expressed in terms of degree Celsius, the heating temperature is desirably a temperature that is at least 200xc2x0 C. but less than 400xc2x0 C., preferably that is at least 225xc2x0 C. but less than 400xc2x0 C., more preferably that is at least 250xc2x0 C. but less than 400xc2x0 C., still more preferably that is at least 300xc2x0 C. but less than 400xc2x0 C. Further, the carrier concentration of the nitride compound semiconductor layer after the heat treatment is at least 1.0xc3x971017 cmxe2x88x923, preferably at least 3.0xc3x971017 cmxe2x88x923, more preferably 5.0xc3x971017 cmxe2x88x923, still more preferably 1.0xc3x971018 cmxe2x88x923.
In the method of heat-treating a nitride compound semiconductor layer or the method of producing a semiconductor device according to the first aspect of the present invention (these methods will be sometimes generally referred to as xe2x80x9cmethod according to the first aspect of the present inventionxe2x80x9d hereinafter) or the method according to the second aspect of the present invention, the heating atmosphere can be an aerial atmosphere (having a pressure that may be atmospheric pressure, a reduced pressure or an elevated pressure). Alternatively, the heating atmosphere can be an atmosphere supplied at least with an oxygen gas, and in this case, the heating atmosphere may be an atmosphere supplied with an oxygen gas alone, an atmosphere supplied with an oxygen gas and a hydrogen gas, an atmosphere supplied with an oxygen gas and steam, or an atmosphere supplied with an oxygen gas, a hydrogen gas and steam. Further, the heating atmosphere may be an atmosphere supplied with an inert gas in addition to these. Alternatively, the heating atmosphere may be an inert gas atmosphere or a reduced-pressure atmosphere having a pressure lower than atmospheric pressure, and in this case, the heating atmosphere may contain steam. The inert gas can be selected from nitrogen (N2) gas, helium (He) gas, neon (Ne) gas, argon (Ar) gas, or mixtures of these gases. When an oxygen gas and a hydrogen gas are supplied, or when an oxygen gas, a hydrogen gas and steam are supplied, the amount ratio of the hydrogen gas in a mixture of the oxygen gas and the hydrogen gas is required to be less than the lower limit (4% by volume) of a combustion range. The supply ratio of the oxygen gas/steam and the amount ratio of the steam in the inert gas atmosphere or the reduced-pressure atmosphere are essentially arbitrary.
According to a third aspect of the present invention for achieving the above object, there is provided a method of heat-treating a nitride compound semiconductor layer, which method comprises heating a nitride compound semiconductor layer doped with a p-type impurity, at a temperature that is at least 200xc2x0 C. but not higher than 1200xc2x0 C., in one atmosphere selected from;
(A) aerial atmosphere,
(B) an atmosphere supplied with an oxygen gas and a hydrogen gas,
(C) an atmosphere supplied with an oxygen gas and steam,
(D) an atmosphere supplied with an oxygen gas, a hydrogen gas and steam,
(E) an inert gas atmosphere containing steam, or
(F) a reduced-pressure atmosphere containing steam and having a pressure lower than atmospheric pressure.
According to a third aspect of the present invention for achieving the above object, there is also provided a method of producing a semiconductor device, which method includes the step of heating a nitride compound semiconductor layer doped with a p-type impurity, at a temperature that is at least 200xc2x0 C. but not higher than 1200xc2x0 C., in one atmosphere selected from;
(A) aerial atmosphere,
(B) an atmosphere supplied with an oxygen gas and a hydrogen gas,
(C) an atmosphere supplied with an oxygen gas and steam,
(D) an atmosphere supplied with an oxygen gas, a hydrogen gas and steam,
(E) an inert gas atmosphere containing steam, or
(F) a reduced-pressure atmosphere containing steam and having a pressure lower than atmospheric pressure.
In the method of heat-treating a nitride compound semiconductor layer or the method of producing a semiconductor device according to the third aspect of the present invention (these methods will be sometimes generally referred to as xe2x80x9cmethod according to the third aspect of the present inventionxe2x80x9d hereinafter), the amount ratio of the hydrogen gas in a mixture of the oxygen gas and the hydrogen gas in the atmosphere supplied with an oxygen gas and a hydrogen gas or the atmosphere supplied with an oxygen gas, a hydrogen gas and steam is required to be less than the lower limit (4% by volume) of a combustion range. Further, the supply ratio of the oxygen gas/steam in the atmosphere supplied with an oxygen gas and a hydrogen gas and the amount ratio of the steam in the inert gas atmosphere or the reduced-pressure atmosphere are essentially arbitrary. Further, the above atmospheres (A) to (E) may be in a state having any one of atmospheric pressure, a reduced pressure and an elevated pressure. Further, the above atmospheres (B), (C) and (D) may contain the above inert gas.
In the method according to the third aspect of the present invention, desirably, the lower limit of the heating temperature is at least 200xc2x0 C., preferably at least 225xc2x0 C., more preferably at least 250xc2x0 C., still more preferably at least 300xc2x0 C. Desirably, the upper limit of the heating temperature is 1200xc2x0 C. or lower, preferably 700xc2x0 C. or lower, more preferably 600xc2x0 C. or lower, still more preferably 500xc2x0 C. or lower, yet more preferably less than 400xc2x0 C. When the upper limit of the heating temperature is set at 700xc2x0 C. or lower, the sharpness of an interface in a superlattice structure is less liable to be disintegrated by the diffusion of atoms (for example, In) constituting the nitride compound semiconductor layer. Further, when the upper limit of the heating temperature is set at 600xc2x0 C. or lower, further at 500xc2x0 C. or lower, further at a temperature lower than 400xc2x0 C., the dissociation of nitrogen atoms from the nitride compound semiconductor layer can be more reliably prevented, and the diffusion of a p-type impurity such as Mg, etc., is less liable to take place. Further, the surface of the nitride compound semiconductor layer is less liable to be oxidized although its degree differs depending upon an atmosphere employed for the heat-treatment.
In the method according to the first, second or third aspect of the present invention, there may be employed a constitution in which a hydrogen-permeable film is formed on the surface of the nitride compound semiconductor layer. In this constitution, examples of a material for constituting the hydrogen-permeable film include so-called hydrogen-occlusion metals such as palladium (Pd) and hydrogen-occlusion alloys. The thickness of the hydrogen-permeable film is not specially limited so long as the dissociation of nitrogen atoms from the nitride compound semiconductor layer by the heat treatment can be prevented. The hydrogen-permeable film can be formed by a physical vapor deposition method (PVD method) such as a sputtering method and a vacuum deposition method or a chemical vapor deposition method (CVD method). For example, a hydrogen-permeable film made of palladium (Pd) is permeable to hydrogen gas at a high temperature, so that it can release hydrogen atoms in the nitride compound semiconductor layer to the heat-treatment atmosphere and can prevent oxidation of the surface of the nitride compound semiconductor layer. Further, palladium can be easily peeled off the nitride compound semiconductor layer and can be also used as a p-side electrode, so that it does not cause the number of steps in the process of producing a semiconductor device, etc., to increase much as compared with the cap layer disclosed in Published Japanese Patent 2540791.
The nitride compound semiconductor layer in the present invention specifically includes GaN, an AlGaN compound crystal, an AlInGaN compound crystal, a BAlInGaN compound crystal, an InGaN compound crystal, InN and AlN. And, it can be formed by a metal organic chemical vapor deposition method (MOCVD method) or a molecular beam epitaxial method (MBE method). The p-type impurity includes Mg, Zn, Cd, Be, Ca, Ba and O.
In the method of producing a semiconductor device according to the first, second or third aspect of the present invention, the semiconductor device includes a semiconductor laser (laser diode, LD), a light-emitting diode (LED) and a transistor such as HBT.
The above heat treatment can be carried out, for example, with an electric oven, various heating apparatuses including heating apparatuses using hot gases such as a hot-air heating apparatus, or an apparatus for irradiation with light or electromagnetic wave such as infrared beam, ultraviolet beam or microwave.
In the method according to the first aspect of the present invention, the heating temperature is set at a temperature that is at least 200xc2x0 C. but less than 400xc2x0 C., namely lower than that employed in any conventional method, and the heat treatment is carried out for a longer period of time than that employed in any conventional method, whereby the resistance of the nitride compound semiconductor layer can be decreased and the nitride compound semiconductor layer can be activated. In the method according to the second aspect of the present invention, the heat treatment is carried out under the conditions where the heating temperature T and the heating time period t satisfy txe2x89xa7100 and the equation (1), whereby the resistance of the nitride compound semiconductor layer can be reliably decreased and the nitride compound semiconductor layer can be reliably activated. In the method according to the third aspect of the present invention, the heat treatment is carried out in the atmosphere containing an oxygen gas or containing steam, so that the lower limit of the heating temperature can be decreased as compared with a conventional method. Further, in the method according to the first, second or third aspect of the present invention, the heating atmosphere is selected from aerial atmosphere or an atmosphere supplied at least with an oxygen gas, so that the resistance of the nitride compound semiconductor layer can be decreased and the nitride compound semiconductor layer can be activated for a short heating time period. This is presumably because water, for example, contained in aerial atmosphere works as a kind of catalyst, or oxygen works as a kind of catalyst, on the surface of the nitride compound semiconductor layer during the heat treatment so that the water or oxygen promotes dissociation of hydrogen in the nitride compound semiconductor layer.